1. Field of the Invention
This invention relates to a method of manufacturing laminates of composite materials and, more particularly, to a method of manufacturing thick laminates of phenolic resin based composite material by a vacuum-bag autoclave type cure process.
2. Description of the Prior Art
Phenolic resin based composites offer superior high temperature and environmental stability without imposing a high temperature cure requirement or the high cost of some other materials such as polyimides and polyquinoxalines. However, the fact that phenolic resins undergo cure with evolution of significant amounts of water has been a deterent to their more widespread use in aerospace applications where they may experience high temperatures in the range of 400.degree.-500.degree. F. A large percentage of phenolic laminates are produced in platen press type operations (compression molding) and involve comparatively thin panels. Platen press operations typically involve short cure cycles (high heating rates) at comparatively high platen pressure. Under these conditions the platen imposed pressure will counterbalance the vapor pressure of the water that is being generated inside the laminate, preventing delamination during the initial cure. Most of the products (e.g. circuit boards) need not withstand high temperatures and thus do not require high temperature postcure. If high-temperature postcure is required, the outgassing process is aided by the fact that with thin laminates any entrapped moisture can usually escape by a diffusional process, preventing the generation of a critical internal gas pressure that would induce delamination.
The fabrication of complex parts by compression molding necessitates the fabrication of costly dies. It is difficult to fabricate thick laminates that will withstand 450.degree.-500.degree. F. temperatures by compression molding because of difficulties in removing the entrapped moisture. The use of a vacuum-bag autoclave type cure process offers for better economy; however, heretofore thick phenolic laminates (60 ply, 0.5 inch or greater) suitable for use in high temperature applications have not been consistently producible by a vacuum-bag autoclave type cure process. Because phenolic resins undergo cure by elimination of water, there has been the widely held belief that, in order to minimize outgassing problems in cure, the preimpregnated fabric should have as low a volatiles content as is permissible within other constraints (e.g., tack and flow). For similar reasons, reliance was placed upon step-cure cycles with the assumption that intermediate temperature dwells were needed to effect outgassing. However, these measures, when applied to the autoclave cure of thick laminates, frequently produce laminates which delaminate during high-temperature post-cure.